Isolated and assembled silver aggregates on the Si(001) surface: the initial stage of film formation.

We examined the dynamics of adsorption and the subsequent growth of submonolayered silver on Si(001) from 100 K to 230 K, using scanning tunneling microscopy and density functional theory. The dynamics is demonstrated to depend on substrate temperature, as described in the following three stages: (I) at 100-140 K, silver is adsorbed as isolated aggregates (regular-Ag4, variant-Ag4 and Ag2), in the absence of single silver adatoms. The spontaneous formation of silver aggregates arises from the hot-atom motion upon the initial impingement of individual silver atoms onto the Si(001) substrate. (II) At 140-190 K, the migration of isolated Ag-aggregates is sufficiently activated, leading to the formation of Ag-chains by surface polymerization. (III) At 190-230 K, there is implication that the Ag-chains become mobile on Si(001), en route to forming patches of 2×2 Ag-films by agglomeration.